69. Binary Subtractor

How do you plan to solve it?

/*Write your code here*/
module sub4_2c (
    input  [3:0] a,
    input  [3:0] b,
    output [3:0] diff,
    output       bout
);
    wire [4:0] sum5 = {1'b0, a} + {1'b0, ~b} + 5'b00001;
    assign diff = sum5[3:0];
    assign bout = ~sum5[4]; // borrow = NOT carry-out
endmodule